Normal saturated steam processing produces one log of reduction (90% "kill") for each 1 to 2 minutes at 15 Lbs. psi. and 250°F 
Sterility is a
measurement and expressed as a probability. Sterility is considered achieved when it reaches what is called a "log of minus 6" rather than as an absolute and is called a "Sterility Assurance Level" (SAL) which can be quantitatively (in numbers) expressed. Let's see what this means. Imagine a glass full of water. Assume for a moment the water represents the microbes and it equals one million microbes (estimated to be on a product). We would call this "level 6", or a "Log of 6" (this is what a log of 6 will mean for our example: equals 1 million).
Imagine we now empty 90% of the
water that is in the glass which would leave 10% of the water. We take what we have left and again empty 90% of that and we get a "log of 4" (there were 100,000 microbes left from the previous step, minus 90% [or  90,000 microbes], and now 10,000 microbes are left in the glass). When we continue again and do this a total number of 6 times we have a "log of 0", which would be one microbe left.
If we want to
continue and empty 90% of what's left (that is, 90% of 1 microbe there is only 1
microbe left) we enter into a "probability" of a
survivor per item. It is obvious that if we have only one microbe we can't
get rid of just 90% of that one microbe. Continuing with our 90% reduction to 2 therefore would mean having one microbe per 100 items, Log3 meaning one per 1,000 etc. Until we reach a log of 6 meaning that only one microbe might survive on a million items presuming that the original microbial load was one million microbes or less. This is called a Sterility Assurance Level (SAL) 10^{6} and it is a number, though it is relative. This is a level of sterilization considered appropriate for implantable items. Acceptable sterilization for
tattoo and piercing means that after you have completed a cycle in your
autoclave there is a probability of one microbe surviving on one million
items.
